Glass substrate for a magnetic disk, a magnetic disk which can be formed with a stable texture

ABSTRACT

A glass substrate for a magnetic disk contains a transition metal. The substrate, which is subjected to formation of zone texturing by irradiating the substrate with a laser beam, has a high mechanical strength even if it is not chemically strengthened. The glass substrate contains a transition metal element and has a transmittance of 10% or less for light in a wavelength range of 300 to 2000 nm. The surface roughness of an information recording surface portion thereof is 2.0 nm or less. In addition to a transition metal element, the glass substrate may include a rare earth metal element.

This application is a continuation of U.S. Ser. No. 09/121,531, filedJul. 24, 1998 abandoned.

BACKGROUND OF THE INVENTION

The present invention is concerned with a glass substrate for a magneticdisk, and were especially, it relates to a magnetic disk with a surfacewhich has no chemical strengthening layer.

Moreover, the present invention relates to a magnetic disk whose surfaceroughness in an information recording portion thereof is sufficientlysmall, and a glass substrate for use in magnetic disk and magnetic diskthat are suitable for a high density recording with high reliability.The present invention also relates to the process of manufacturing aglass substrate and a magnetic disk device.

Texturing of a glass substrate of a magnetic disk by using a laser hasbeen attempted recently. While this method involves a technique used onmetal substrates, such as Ni—P, this technique also maybe applied toglass. As described in Patent pre-publication 7-182655 and Patentprepublication 9-138942, laser texturing techniques using an ultravioletlight laser of 266 nm or a carbon dioxide laser that has a longwavelength of 10.6 μm have been used for ordinary glass having atransparency to light in the range from infrared to near ultraviolet.

In the above-described glass system, the substrate is reinforced by achemical strengthening layer, and it is also known that textureformation maybe easily performed by compression of the substratesurface. (Ref. A. C. Tam et. al., IEEE(1997)).

SUMMARY OF THE INVENTION

As a recording medium for a note book type personal computer, a 2.5″glass substrate with a high surface smoothness is installed in amagnetic disk device. The glass substrates implemented at present arechemically strengthened glass substrates and crystallized glasssubstrates. However, lowering of the traveling height of the magnetichead is necessary to increase the recording capacity or recordingdensity per unit area of the magnetic disk. Therefore, a magnetic diskwith a smoother recording surface is being developed.

When the magnetic disk is rotating, the magnetic head floats on therecording surface of the disk, and, when the disk stops, the head landson and contacts the disk. Thus, sticking may occur when the face wherethe head lands is too smooth. Therefore, the landing point of the headson the disk surface must have a proper roughness. For the reason, atexture processing that makes all recording surfaces of the magneticdisk have a proper roughness has been adopted. However, in view of therequirement to provide a small traveling height, the texture should notbe formed on the entire recording surface, but should be formed only ina zone (CSS zone; Contact Start Stop Zone) that is formed where the headlands (zone texturing).

A method of etching, sputtering or sol-gel coating only a CSS zone aftermasking the recording surface has been attempted as a method ofeffecting zone texturing. However, with these methods, the masking isnot sufficient in some cases, and so the recording surfaces become roughto some extent, which reduces the available recording area. To solvethis problem, the texture is formed by using a laser. There is a methodof providing a laser texture, which is described in Japanese patentLaid-open print No. 7-182655. However, with such method, it wasdifficult to form the texture with a stabilized configuration and width,because the method uses a carbon dioxide gas laser and the outputstability of a carbon dioxide gas laser is not satisfactory. Since thewavelength of the carbon dioxide gas laser is long (10.6 μm), the spotdiameter becomes large. Therefore, it is difficult to form a small sizedtexture.

Sufficiently small bumps that constitute a texture are formed by usingultraviolet light in the method disclosed in Japanese patent Laid-openprint No. 9-138942. In this specification, the term bump is used todescribe each of a plurality of small projections that constitute thetexture of the disk. Because the depth of focus is shallow in thismethod, focusing on the substrate surface is difficult, so that controlof the bump height with a high degree of accuracy is difficult. Becauseultraviolet light was absorbed in the ordinary lens system, the loss waslarge. This produced a difficulty and danger to operators as well,because ultraviolet light cannot be directly observed with the nakedeye.

The mechanical strength of the chemically reinforced substrate is highand the formation of the laser texture is easy to achieve in thissubstrate, but there is a fear that alkali ions with a large iondiameter that are introduced into the glass substrate by the chemicalstrengthening will move to the substrate surface. As a result, thepeeling of a magnetic film from the substrate or sticking may arise. Acrystallized glass substrate is conceivable as an example of otherstrengthened substrates. In case a crystallized glass substrate is used,however, the smoothness of the recording surface is not sufficient, andso it is difficult to obtain a high recording density.

In view of the above, an object of the present invention is to provide aglass substrate with a high chemical stability and to obtain a glasssubstrate for a magnetic disk for forming a stable laser texture whichis suitable for use as a high recording density magnetic disk as well.

Another object of the present invention is to obtain a glass substratefor a magnetic disk having a high mechanical strength with goodreliability and which has no chemically reinforced layer.

A further object of the present invention is to provide a magnetic diskhaving high reliability and a high recording density and a magnetic diskdevice using the same.

The invention employs the following measures to achieve the aboveobjects. A glass substrate for the magnetic disk of the presentinvention has an information recording surface to record information onat least a part of the surface thereof, but it does not have a chemicalstrengthening layer on the surface. The surface roughness Ra of theinformation recording surface is 2.0 nm or less, and there exists awavelength zone in which the transmittance of light becomes 10% or lessin the wavelength range of from 300 nm to 2000 nm. In other words, theglass substrate should have a transmittance of 10% or less to the laserlight being used.

At least one transition metal element, which is selected from the groupconsisting of titanium, vanadium, chromium, manganese, cobalt, nickel,copper, gold and silver, is contained in the glass substrate. Thetransition metals are used for absorbing laser light and reducing thetransmittance of the glass composition to laser light. When thetransition metal element is cobalt, 1 to 30% by weight of cobalt ispreferably contained on the basis of the conversion as CoO. In thiscase, using CoO, the wavelength zone for effecting laser texturing is inthe range of from 450 to 700 nm.

The composition of the glass substrate for the magnetic disk of thepresent invention should contain a rare earth metal, which increases themechanical strength of the glass substrate. An example of a preferableglass composition is: SiO₂: 50 to 80 weight %, B₂O₃: 0 to 15 weight %,R₂O: 0 to 20 weight % (R=alkali metal element), Ln₂O₃: 0 to 10 weight %(Ln=rare earth element), Al₂O₃: 0.5 to 15 weight %, CoO: 1 to 30 weight% on the basis of oxide conversion.

The magnetic disk of the present invention comprises a circular ordisk-shaped glass substrate and an information recording film formeddirectly or through another layer on the surface of the substrate. Thesubstrate surface has a non-information recording area and aninformation recording area that are formed in concentric relation to theperiphery of the substrate. The surface roughness Ra of the informationrecording surface is 2.0 nm or less, and the maximum surface roughnessRmax is 5 nm or less. The laser texture is formed in the non-informationrecording area. The structure of the texture should be regularly formed.The mean height of the bumps that constitute this texture shouldpreferably be 10 nm to 25 nm.

A chemical strengthening layer does not essentially exist in the surfacepart of the glass substrate. The substrate has a transmittance of 10% orless to monochromic light or laser light, such as light having awavelength of 300 to 2000 nm.

The manufacturing process of the magnetic disk of the present inventioncomprises the following steps. A substrate, which has no chemicalstrengthening layer in the surface thereof and has a surface roughnessRa of 2.0 nm or less, is used. The texture is formed in thenon-information recording area of the surface of the glass substrate byirradiating it with a laser beam having a wavelength of 300 to 2000 nmso as to form bumps having a mean height of 10 nm to 25 nm. When theglass substrate contains cobalt, the preferable wavelength to be used is450 to 700 nm. The magnetic disk device of the present inventioncomprises a magnetic head, a magnetic disk, a spindle motor to rotatethe magnetic disk, and a motor for changing the position of the magnetichead in a direction parallel with the surface of the magnetic disk.

The magnetic disk has a glass substrate and a magnetic film formeddirectly or through another layer on it. The disk surface has anon-information recording area and an information recording area. Duringrotation of the magnetic disk, the traveling height of the magnetic headon the information recording surface is 30 nm or less, the surfaceroughness Ra of the information recording surface is 2.0 nm or less, andRmax is 5 nm or less.

A chemical strengthening layer does not exist on the surface part of theglass substrate. The substrate has a transmittance of light of 10% orless in the wavelength range of 300 to 2000 nm.

According to the present invention, because there is no chemicalstrengthening layer on the surface, a glass substrate for a magneticdisk that is excellent in chemical stability can be obtained. Moreover,a glass substrate for a magnetic disk on which a laser texture can beformed stably in a predetermined noninformation recording area isobtained. Therefore, a glass substrate for a magnetic disk with a highrecording density is obtained. When rare earth elements are added to thesubstrate, a glass substrate having a high mechanical strength and ahigh reliability are obtained even though it does not have a chemicalstrengthening layer. Thus, a magnetic disk and a magnetic disk devicewith a high recording density and a high reliability can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a glass substrate for a magnetic disk accordingto one example of the present invention.

FIG. 2 is the graph that shows wavelength dependence of thetransmittance of the glass substrate of the present invention.

FIG. 3 is a sectional view of the shape of a bump that forms part of alaser texture manufactured on the glass substrate surface according tothe present invention.

FIG. 4 is the graph that shows the height of a bump relative to thelaser power per pulse.

FIG. 5 is a sectional view representing an magnetic disk of the exampleof the present invention.

FIG. 6 is the perspective view of a magnetic disk apparatus according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in detail by reference to thedrawings.

EXAMPLE 1

The glass compositions of nineteen glass substrates for magnetic discsmanufactured in accordance with the present invention are shown in Table1.

TABLE 1 Composition (weight %) No. SiO₂ Na₂O Li₂O B₂O₃ Al₂O₃ CaO MgOGd₂O₃ Er₂O₃ CoO NiO FeO remarks 1 72.5 14.0 — — 0.9 8.0 4.1 3.0 — 0.5 —— 2 71.5 13.0 — — 0.5 7.0 3.0 3.0 — 5.0 — — 3 72.0 14.0 — — 0.9 8.0 4.13.0 — 1.0 — — 4 51.8 10.0 — — 1.0 5.7 2.9 3.0 — 28.6 — — 5 69.5 12.0 — —0.5 6.0 2.0 3.0 — 10.0 — — 6 45.3 8.8 — — 0.9 5.0 2.6 3.0 — 37.4 — — * 772.5 14.0 — — 0.9 8.0 4.1 3.0 — — 0.5 — 8 71.5 13.0 — — 0.5 7.0 3.0 3.0— — 5.0 — 9 72.5 14.0 — — 0.9 8.0 4.1 3.0 — — — 0.5 10 71.5 13.0 — — 0.57.0 3.0 3.0 — — — 5.0 11 68.5 10.0 — 5.0 1.0 7.0 5.0 3.0 — 0.5 — — 1268.0 8.0 — 4.0 1.0 6.0 5.0 3.0 — 5.0 — — 13 68.0 10.0 — 6.0 1.0 7.0 4.03.0 — 1.0 — — 14 68.0 8.0 — 5.0 1.0 6.0 4.0 — 3.0 5.0 — — 15 66.0 6.0 —3.0 1.0 5.0 4.0 — 10.0 5.0 — — 16 66.0 5.0 — 3.0 1.0 5.0 3.0 — 12.0 5.0— — * 17 69.0 10.0 — 4.0 1.0 6.0 5.0 — — 5.0 — — 18 71.5 13.0 — — 0.57.0 3.0 — — — — 5.0 ** 19 71.5 3.0 11.0 — 8.5 0.5 0.5 — — 5.0 — — *:devitrified, **:chemical strengthened, ***:crystallized

First, a method of manufacturing a glass substrate having a compositionas shown in Table 1 will be described. After weighing, combining andmixing the material powders of determined quantities in a crucible, thecomposition was melted at 1600 degrees centigrade in an electricfurnace. After the composition was sufficiently melted, a stirrer wasinserted in the molten glass, and the molten glass was stirred for about1 hour. A glass block was obtained by pouring the molten glass into ajig after taking out the stirrer and then allowing it to stand still for30 minutes. The glass block was reheated to a temperature in theneighborhood of the glass transition point and was annealed to removestress. The obtained glass block was sliced with a cutter to obtain adisk configuration having a thickness of about 1.5 mm. Each disk has aninner periphery and an outer periphery in concentric relation. The innerperiphery and the outer periphery of the disk were machined to level thesurface with a diamond tool. Then, both sides of the substrate werelapped and polished, with a result that the glass substrates for themagnetic discs were obtained.

FIG. 1 is a plane view of the manufactured glass substrate for themagnetic disk. In FIG. 1, the magnetic disk includes a glass substrate 1having an inner periphery 2 for chucking with a chuck, a recordingsurface 3, and a CSS zone 4. In this example, the thickness of thesubstrate was 0.635 mm. The inner periphery was 20 mm in diameter. Theouter periphery was 65 mm in diameter. With the common center of theinner and outer peripheries designated 0, the CSS zone 4 was provided inan area in the range of 13 to 15 mm from the center of the disk, and alaser texture was formed therein. In Table 1, substrates No. 1 to No. 16have glass compositions containing a rare earth element and cobaltoxide. Substrate No. 17 has a glass composition containing no rare earthelement. Substrate No. 18 is formed of a chemical strengthened glassobtained by chemical strengthening the glass of substrate No. 10.Substrate No. 19 is a crystallized glass containing cobalt oxide. Theglass compositions of substrates No. 6 and No. 16 did not become glass,and they were diversified immediately after casting. Therefore, theseglasses are not desirable for use as substrates.

Table 2 shows the transmittance of light at various laser wavelengths ofthe nineteen glass substrates shown in Table 1, the three point bendingstrength of the base glass materials, the state of the laser textureformation, the water resistance of the substrate, the roughness Ra ofthe recording surface and the Vickers hardness of the substrate surface.

TABLE 2 266 nm 532 nm 1064 nm Trans- Texture Trans- Texture Trans-Texture Water Rough- Three points mittance forma- mittance forma-mittance forma- resis- ness Micro Vickers bending strength Exs. (%) tion(%) tion (%) tion tance (Ra/nm) hardness (Hv) (σ/XPa) remarks 1 0.12 Δ1.82 Δ 72.0 X ◯ 1.000 679 212 2 0.12 Δ 0.06 ◯ 2.00 Δ ◯ 0.895 680 222 30.15 Δ 1.03 ◯ 65.0 X ◯ 0.902 679 215 4 0.20 Δ 0.02 ◯ 0.06 Δ ◯ 1.960 682221 5 0.18 Δ 0.04 ◯ 1.00 Δ ◯ 1.502 686 261 6 — — — — — — — — — — * 70.17 Δ 40.0 X 70.0 X ◯ 0.895 679 225 8 0.16 Δ 0.12 Δ 10.0 Δ ◯ 0.902 682231 9 0.07 Δ 90.0 X 70.0 X ◯ 0.878 680 284 10 0.03 Δ 36.0 X 6.00 Δ ◯0.899 680 241 11 0.20 Δ 4.00 Δ 70.3 X ◯ 0.854 679 201 12 0.12 Δ 0.09 ◯2.60 Δ ◯ 0.952 685 221 13 0.17 Δ 1.06 ◯ 64.5 X ◯ 0.874 679 210 14 0.18 Δ0.08 ◯ 2.50 Δ ◯ 0.896 672 288 15 0.16 Δ 0.08 ◯ 2.60 Δ ◯ 0.874 686 315 16— — — — — — — — — — * 17 0.16 Δ 0.07 ◯ 2.60 Δ ◯ 0.875 623 160 18 0.06 ◯0.06 X 2.00 Δ X 0.765 711 506 ** 19 0.17 Δ 0.03 ◯ 1.00 Δ X 3.242 742 322*** *:devitrified, **:chemical strengthened, ***:crystallized

In this example, the fundamental wavelength of the pulse YAG laser (1064nm), the second harmonic generation laser (532 nm) and the fourth orderharmonic generation laser (266 nm) were used for texture formation. Thelaser power per pulse was about 2.0 μJ. The frequency of the laser pulsewas 20 kHz. The transmittance was measured by a spectrophotometer.Whether the texture was formed or not was judged by observing thesubstrates with an optical microscope after laser irradiation. It wasalso judged by evaluating the bump configuration on the substrate duringexamination with AFM (Atomic Force Microscopy) to determine there weretraces. In Table 2, in the texture columns, circle (O) stands for thecase where a good texture was formed because the traces remained, thebump configuration as determined with AFM observation was good and thedispersion of the height was small; a triangle (Δ) stands for the casewhere, while the traces remained, the bump configuration was not goodand the control of the height was not satisfactory; and a cross mark(×)stands for the case where the traces did not remain as determined byobservation with an optical microscope or AFM.

The water resistance was evaluated by measurement of the quantity ofeluted components from the glass composition in the demineralized water.Each glass substrate was dipped for 24 hours at 70° C. in 80 mldemineralized water. In Table 2, in the water resistance column, Ostands for the acceptable case, and X stands for not acceptable cases.The surface roughness was evaluated by using a roughness tester.

The Vickers hardness of the substrate surface was evaluated by the sizeof the pressure scar. A diamond indenter was applied to the surface ofthe substrate under the condition of 100 gf-15s.

The three point bending strength was evaluated by using chamfered 0.635mm×4 mm×40 mm specimens. The crosshead speed was set at 0.5 mm/minute.

The transmittance of all glass compositions was low when the laser lightwavelength was 266 nm. While in the glass compositions of substrate Nos.1 to 17 that are not chemically strengthened, traces of the texture wereobserved, sufficient control of height and configuration was difficult.While the traces of the texture of the comparative glass composition ofsubstrate No. 19, which is crystallized glass, could be confirmed, thedispersion of the height was large. So a stable texture could not beformed. On the other hand, in chemical strengthened glass containingiron oxide (comparative example No. 18), a stabilized texture could beformed. A texture having a stable configuration was obtained in the sodalime glass containing 1.0 weight % or more of CoO (No. 2-5) by use of a532 nm wavelength laser.

A texture having a stable configuration was also formed by borosilicateglass (No. 12-15, and 17). It was possible to form the texture stably inthe crystallized glass of comparative example No. 19. In case of thechemically strengthened glass of comparative example No. 18, traces ofthe texture could not be found when a 532 nm laser was used.

In case of the soda lime and borosilicate glasses containing 0.5 weight% of CoO (No. 1, 11), though a texture formation could be confirmed, astable texture formation could not be observed. On the other hand, inglass containing NiO, the texture formation could not be confirmed whenthe NiO content was 0.5 weight % (No. 7). In the glass containing 5.0weight % NiO (No. 8), formation of the texture was observed. However, itwas difficult to obtain a stabilized texture. In case of the fundamentalwave, texture formation was recognized in glass containing 5 weight % ormore of NiO and CoO (No. 2, 4, 5, 6, 8, 12,14, 15, 16,17, 18 and 19).The control of the configuration of the texture was not so good and theheight control of the obtained texture was difficult. In other glasses,the traces of the texture could not be recognized. The water resistancewas satisfactory in the glasses of substrates No. 1-No. 17.

On the other hand, in the chemically strengthened glass of substrate No.18, quite large amounts of K, Na and Ca, which were used for chemicalstrengthening the glass composition, were eluted in water, so the waterresistance was not sufficient. In case of the crystallized glass ofsubstrate No. 19, Li eluted, so that the water resistance was notsufficient. While the surface roughness was 2 nm or less in the glassesof substrates Nos. 1-18, it was 3.2 nm in the crystallized glass ofsubstrate No. 19. The micro Vickers hardness was around 680 in theglasses of substrates Nos. 1-15 containing rare earth elements. On theother hand, it was 623 in the borosilicate glass of substrates No. 17containing no rare earth element, which value was not good. In addition,the three point bending strength tests brought about the same result asthose of the micro Vickers hardness tests. In the cases of thechemically strengthened glass and the crystallized glass, the mechanicalstrength was sufficiently high.

The glass substrates for the magnetic recording disk were subjected toevaluation of the possibility of texture formation by laser, chemicalstability, surface roughness and mechanical strength. At first, when thelaser wavelength is 266 nm, which is fourth order generation light, astable texture could be formed only in case of the chemicallystrengthened glass, but it was difficult to form it in other glasses.Therefore, chemical strengthening becomes a condition of the stableformation of the laser texture for this wavelength laser.

On the other hand, it was difficult to produce a magnetic recording diskwith high reliability, because there is a problem in the chemicalstability of the chemical strengthening of the glass. Therefore, it isdifficult to form a stable laser texture in the case of a 266 nmwavelength laser. However, the laser textures could be formed stably bya 532 nm laser from soda lime glass and borosilicate glass, eachcontaining 1.0 weight % or more of CoO. Especially, in borosilicateglass including a rare earth element, the chemical stability andmechanical strength were sufficient. Therefore, a magnetic disk having ahigh reliability could be obtained. Though it was possible to form thetexture stably, using a 532 nm laser, in the crystallized glass, thesurface roughness was large. Therefore, as will be described later inexample 2, there was a problem in the characteristics of the magneticdisk. While at the fundamental wave of 1064 nm, there are glasscompositions with which it is possible to form a texture, the dispersionof the characteristics is large. It is also difficult to form a stabletexture and to control the bump height in the texture.

It was necessary to add a transition metal corresponding to the laserwavelength being used. Fe was contained in the case of 266 nm, Co in thecase of 532 nm, and Ni in the case of 1064 nm. It was also possible toform a laser texture only in the substrate having chemical strengtheningin the case of a 266 nm laser, but the stable texture was difficult toform in case of a 1064 nm laser. From the above examination, it wasconcluded that in order to obtain a magnetic disk with a highreliability and to form the laser texture stably, it is desirable toform the texture by using a laser having a wavelength of 532 nm forglass containing CoO and a rare earth element.

Then, the transmittance of the glass substrate to light in thewavelength for forming the texture stably was checked by using Table 2.With respect to substrate No. 8, it was found that the configuration wasunstable and the transmittance to laser light of 1064 nm was 10%. But,it was possible to form a texture. Formation of a texture was difficultwhen the transmittance was over 10%.

If the transmittance with respect to each wavelength is 2.0% or less, amore stable texture could be obtained. From the above, when thetransmittance at each laser wavelength is 10% or less, a texture ofstable configuration could be formed with height control when thetransmittance was 2% or less.

FIG. 2 shows the wavelength dependency of the transmittance of the glassof substrate No. 12. When Co oxide is contained in the glass, it wasproved that there was a large absorption of light at such wavelengths as450 nm to 700 nm. As the CoO content increases, the transmittance in theabsorption edge neighborhood declined. Then, it was proved that, whenthe CoO content is 13 weight %, the transmittance with respect towavelengths of 450 nm and 700 nm became almost 0. Even if CoO iscontained in larger amounts, the transmittance curve did not change.Therefore, as for the optical characteristics of the glass, an amountexceeding 13 weight % of CoO is excessive.

From Table 2, it is seen that a stable laser texture was formed when thetransmittance was 2% or less. In this system, the requirements weresatisfied in the range of 450 nm to 700 nm when the content of CoO washigh. Therefore, it was possible to form a laser texture using any laserwith a wavelength in this range. In addition, by adding copper,chromium, manganese, vanadium, titanium, etc. to the glass compositions,enough absorption of light can be effected over the range of from thenear ultraviolet area to the infrared area. Light in the range of 300 nmto 400 nm was absorbed by adding vanadium. By adding an oxide ofchromium, light in the range of 300 nm to 450 nm was absorbed. Light inthe range of 400 nm to 700 nm was absorbed by adding nickel. Light inthe range of 450 nm to 700 nm and 1000 nm to 2000 nm was absorbed byadding cobalt. Light in the range of 550 to 1500 nm was absorbed byadding copper. Light in the range of 450 nm to 800 nm was absorbed byadding titanium.

It was possible to form a laser texture in these wavelength areas. Thelaser texture could be formed by adding transition metal elementscorresponding to each wavelength between 300 nm and 2000 nm. In the casewhere gold mad silver are made into metal colloids in the glass,absorption of light at a specific wavelength was observed, and texturecould be formed in this wavelength range. Because a rare earth metalelement also absorbs light, a similar effect can be expected. Theabsorption by a rare earth element is very sharp because of the F—Ftransition of the inner shell electrons, so that laser textureprocessing was possible only in the very narrow wavelength range. Theconfiguration of a bump in that laser texture manufactured in thisexample is shown in FIG. 3. In FIG. 3, 1 denotes a glass substrate, and5 denotes a bump which forms part of the texture. This was obtained byobserving the texture obtained in the glass of substrate No. 12 withAFM.

The bump configuration obtained in this example has a dome form as shownin this figure. Observation with a SEM (Scanning Electron Microscope)revealed that the glass was melted once, and was quenched so as tore-solidify. At this time, the rate of volumetric shrinkage of the bumpis smaller than that of the substrate that was manufactured by slowcooling or annealing, so that the volume of the bump becomes larger.Thus, the bump portion swells to form a bump of the dome type. By usingthe glass of substrate No. 12, the effect of laser power per pulse onthe bump height was checked. The result is shown in FIG. 4. The bumpheight greatly changes as the laser power changes. The bump height wasabout 15 nm in the case of 2 μJ of laser output, and 100 nm at the timeof 5 μJ. The height of 1000 nm was obtained in the case of 10 μJ ormore. All bumps had the dome configuration as shown in FIG. 3. The bumpdiameter was about 5 to 10 μm in all cases. When the frequency of thepulse was 20 kHz, the pitch between bumps was about 10 μm. Then, propercontents of the additives in the component of each glass substrate werestudied. Tables 3-1 and 3-2 show the compositions of the examinedglasses, the micro Vickers hardness (Hv), the eluted quantities of thecomponents in the water resistance tests, the surface roughness (Ra/nm)of the recording surface, the presence or absence of the laser formationof texture and the presence or absence of a glass formation.

TABLE 3 Laser Composition (weight %) Micro Vickers Water Roughnesstexture Glass No. SiO₂ Na₂O B₂O₃ Al₂O₃ CaO Gd₂O₃ CoO hardness (Hv)resistance (Ra/nm) formation formation 20 50.2 15.0 12.4 14.4 — 3.0 5.0653 ◯ 1.000 ◯ ◯ 21 49.6 15.4 12.5 14.5 — 3.0 5.0 652 ◯ 0.895 X X 22 78.87.1 4.1 2.0 — 3.0 5.0 743 ◯ 0.878 ◯ ◯ 23 80.4 5.7 3.7 2.2 — 3.0 5.0 760◯ 0.899 ◯ X 24 59.0 10.6 10.2 12.2 — 3.0 5.0 680 ◯ 0.854 ◯ ◯ 25 60.6 9.89.8 11.8 — 3.0 5.0 734 ◯ 0.856 ◯ ◯ 26 60.2 16.3 14.5 1.0 — 3.0 5.0 732 ◯0.895 ◯ ◯ 27 60.1 15.6 15.7 0.6 — 3.0 5.0 720 X 0.902 ◯ ◯ 28 66.1 18.63.7 3.6 — 3.0 5.0 680 ◯ 0.921 ◯ ◯ 29 65.9 18.2 4.5 3.4 — 3.0 5.0 684 ◯0.895 ◯ ◯ 30 60.3 20.2 6.5 5.0 — 3.0 5.0 724 X 0.944 ◯ ◯ 31 60.7 19.26.8 5.3 — 3.0 5.0 732 ◯ 0.952 ◯ ◯ 32 74.8 7.2 5.0 5.0 — 3.0 5.0 742 ◯0.874 ◯ ◯ 33 74.8 6.5 5.4 5.3 — 3.0 5.0 748 ◯ 0.896 X ◯ 34 62.1 14.514.8 0.6 — 3.0 5.0 665 ◯ 0.874 ◯ ◯ 35 62.2 14.6 14.9 0.3 — 3.0 5.0 652 ◯0.956 ◯ ◯ 36 60.9 19.4 6.9 4.8 — 3.0 5.0 662 ◯ 0.875 ◯ ◯ 37 66.0 10.09.0 8.0 2.0 — 5.0 625 ◯ 0.765 ◯ ◯ 38 65.8 9.8 8.8 7.8 1.8 1.0 5.0 692 ◯0.902 ◯ ◯ 39 65.5 9.0 8.5 7.5 1.5 3.0 5.0 732 ◯ 0.600 ◯ ◯ 40 65.3 8.68.3 7.3 1.5 4.0 5.0 725 ◯ 0.952 ◯ ◯ 41 65.1 8.2 8.1 7.1 1.5 5.0 5.0 736◯ 1.054 ◯ ◯ 42 64.6 7.2 7.6 6.6 1.0 8.0 5.0 740 ◯ 1.455 ◯ ◯ 43 63.8 7.66.8 5.8 1.0 10.0 5.0 745 ◯ 1.562 ◯ ◯ 44 63.6 7.2 6.6 5.6 1.0 11.0 5.0760 ◯ 1.899 ◯ X

As seen in Tables 3-1 and 3-2, the micro Vickers hardness and durabilitywas evaluated by use of the above method. The laser texture was formedon a glass substrate of 2.5″ by using 532 nm laser light. Like substrateNo. 21 in Table 3-1 and the glass of substrate No. 6 in Table 1, thechemical stability of the glasses was low in case of a SiO2 content ofless than 50.0 weight %, so it did not form a glass. When a glass ofSiO2 content of 50.0 weight % or more like the glass of substrate No. 4in Table 1 and No. 22 in Table 3-1, a stable glass could be obtained.

While a stable glass could be obtained in substrate No. 22 in Table 31,when the SiO₂ content exceeded 80% like the glass of substrate No. 23,the viscosity of the glass became very high. It became a glass with alot of cords and pores so that melting of the glass composition wasdifficult. From the above, it has been determined that the SiO₂ contentof 50 to 80% is preferable. While a stable glass was obtained from thecomposition of substrate No. 24 in Table 3-1, the micro Vickers hardness(Hv) was low, and the mechanical strength was not sufficient. On theother hand, the Hv was 734 in the glass of substrate No. 25, and so asufficient mechanical strength was obtained. An SiO₂ content of 60weight % or more is further desirable to obtain a high mechanicalstrength.

On the other hand, though a stable glass could be formed in substrateNo. 22, it was difficult to form a laser texture because of its highcharacteristic temperature. From the above, it is seen that it isdesirable that the SiO₂ content is 60 to 75 weight % to obtain asufficient mechanical strength and water resistance. In this range, thesurface roughness can be made small, and the laser texture is formed inthe glass effectively.

The laser texture could be formed without adding B₂0₃ like the glassesof substrates Nos. 1-10. But, when B₂0₃ exceeds 15 weight % likesubstrate No. 27 of Table 3-1, B₂0₃ was eluted in the water resistancetest, and chemical stability declined. Therefore, if the B₂0₃ content is15 weight % or less, a stable glass can be obtained. Like the glass ofsubstrate No. 30 of Table 3-1, when the content of the sum of alkalioxides exceeds 20 weight %, the glass showed a large amount of alkalielusion.

If the sum quantity of added alkali oxides is 20 weight % or less, thealkali elusion quantity could be sufficiently small like the glass ofsubstrate No. 31. Therefore, the content of the alkali elements shouldbe 20 weight % or less. Because the sum quantity of the alkali oxides inthe glasses of substrates No. 33 and No. 23 is 7.0 weight % or less, thecharacteristic temperature was too high, and it was difficult to form atexture using the laser. The alkali content was 7.0 weight % or more insubstrates No. 32 and No. 22, and it was possible to form a textureusing the laser. From the above, it is seen that the sum quantity of analkali metal oxide should be 7 to 20 weight % .

When the rare earth oxides are contained in the glass composition, themechanical strength can be improved without chemical strengthening. Likesubstrate No. 16 in Table 1 and substrate No. 44 in Table 3-2, when therare earth elements exceed 10 weight %, they react with transition metalelements to cause devitrification. Therefore, the content of rare earthoxides should be 10 weight % or less. When the content of CoO is over 30weight %, the glass was diversified like the glass of substrate No. 6.It was difficult to form a laser texture when the CoO content was lessthan 1.0 weight %, like substrate No. 1 and No. 11. Therefore, thecontent of CoO should preferably be 1 to 30 weight %. When the CoOcontent is over 13 weight %, the amount of CoO becomes excessive fromthe saturation condition of absorption of light, and a preferable CoOcontent is 1 to 13 weight %.

EXAMPLE

A magnetic disk and a magnetic disk device were manufactured using theglass substrate manufactured in example 1, and the characteristics wereevaluated.

FIG. 5 shows a schematic diagram of a section of the manufacturedmagnetic disk, which includes a glass substrate 1, a precoat film 6 thatconsists of materials containing Cr, a base film 7 consisting of theCrTi materials, a magnetic film 8 consisting of the Co—Cr—Pt, anovercoat 9 consisting of carbon, and a lubricant 10. These films weremanufactured on both sides of the substrate. In this example, thesubstrate No. 12 and the glass substrate No. 19 made of crystallizedglass were used as a glass substrate, and a laser zone texture wasformed in an area 13 to 15 mm from the center.

The texture was formed by using a laser of 532 nm. The bump height waschanged in the range of from 5 nm to 40 nm by changing the power perpulse of the laser. After washing and drying the manufactured glasssubstrate, the magnetic film 8 was formed by sputtering. In thisexample, the pre-coat film was 22 nm, the foundation film was 25 nm, themagnetism film was 19 nm, and the protection film was 29 nm.

The lubrication material was applied by dipping the disk in a lubricantsolution to obtain the magnetic disk. The schematic diagram of amagnetic disk device is shown in FIG. 6. In FIG. 6, 11 denotes amagnetic disk, 12 denotes a rotation axis of the magnetic disk, 13denotes a spindle motor, 14 denotes a magnetic head, 15 denotes arotation axis of the magnetic head, and 16 denotes the electrical signaloutput terminal of an electric system.

The magnetic disk 11 is supported for rotation on axis 12, and themagnetic disk is rotated by driving the disk using the spindle motor.The magnetic head is supported on the head rotation axis and the radialposition on the disk is determined by rotation of the head supportaround the head rotation axis. In this example, six magnetic disks weremounted in the device, and magnetic heads were mounted on the both sidesof each disk to provide twelve heads in total. The thickness of thecasing was 12 mm.

The traveling height of the magnetic head was controlled by the springhardness of the arm that supports the magnetic head and theconfiguration of the magnetic head, which were controlled. The travelingheights of the magnetic head were set to 20 nm, 30 nm and 40 nm,respectively. The traveling height of the head in the magnetic diskequipment manufactured by using substrates of various coarseness and therecord reproduction characteristics of the magnetic disk device areshown in Table 4. The Ra and Rmax of each glass substrate are alsoshown.

TABLE 4 Travel- ingheight (nm) Ra Rmax No. 20 30 40 (nm) (nm) 12 ◯ ◯ Δ0.652 3.0 ◯ ◯ Δ 0.821 3.3 ◯ ◯ Δ 0.952 4.0 Δ ◯ Δ 1.241 4.4 Δ ◯ Δ 1.9434.6 X Δ Δ 2.241 8.4 X Δ Δ 2.896 12.2 X Δ Δ 3.358 30.0 19 X X Δ 3.24230.0

C/N of the read signal was not sufficient when the head traveling heightwas 40 nm in any substrate. When the head traveling heights were 20 nmand 30 nm, the characteristics of surface roughness of the substrateswere difficult. When the traveling height was 30 nm, the read-writecharacteristics of the substrate with 2 nm or less of Ra was sufficient.

The read-write characteristics of the substrate having an Ra of 2 nm ormore were not satisfactory. While good read-write characteristics wereobtained in the substrate having an Ra of lnrn or less, when thetraveling height was 20 nm, good characteristics were not obtained inthe substrate having an Ra of over 2 nm. A high C/N was obtained withthe magnetic disk equipment using the glass substrate No. 12, and goodcharacteristics were shown. When the crystallized glass substrate No. 19was used, the head was damaged. This is because the Rmax of thissubstrate is too large (30 nm).

High C/N was obtained, when the head traveling height was 30 nm or less.When the Ra of the disk is 2.0 nm and Rmax is 5 nm or less, a travelingheight of 30 nm or less could be realized. In addition, good read-writecharacteristics were obtained with a traveling height of 20 nm when Rais 1 nm or less. The result of a reliability test for each bump heightof the texture of the magnetic disk device using the glass substrate No.12 is shown in Table 5.

traveling height (nm) No. 5 10 15 20 25 30 12 X ◯ ◯ ◯ ◯ X

The head traveling height was 30 nm. A start and stop operation wasrepeated about 10⁴ times. In Table 5, X stands for a default O and Ostands for no default. When the bump height is 5 nm, the head sometimesbecame stuck to the magnetic disk. No default, such as head damage,occurred in the range of from 10 nm to 25 nm of bump height. On theother hand, when the head traveling height is 30 nm or more, a default,such as head damage, was observed. From the above, it is seen that it isdesirable that the height of the bumps should be from 10 nm to 25 nm.

What is claimed is:
 1. A glass substrate for a magnetic disk,comprising: a glass substrate having an information recording surfacewhich is formed in the surface of the glass substrate, and a textureformed in the surface of the substrate besides the information recordingsurface, wherein the surface part of the glass substrate issubstantially free from a chemical strengthening layer, and wherein saidglass substrate contains a rare earth element which increases themechanical strength of the glass substrate, the rare earth element beingselected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Er, Tm, Yb and Lu and being contained in an amount of 3 to 10 weight% on the basis of conversion to Ln₂O₃ (Ln=rare earth element), and atransition metal oxide.
 2. The glass substrate for a magnetic diskaccording to claim 1, wherein the transition metal oxide is an oxide ofa transition metal element selected from the group consisting oftitanium, vanadium, chrome, manganese, cobalt, nickel, copper, gold andsilver.
 3. The glass substrate for a magnetic disk according to claim 1,wherein the transition metal oxide is CoO and is contained in an amountof 1 to 30 weight %, and wherein said laser has a wavelength in therange of 450 to 700 nm.
 4. A glass substrate for a magnetic diskaccording to claim 1, wherein the glass substrate contains SiO₂: 50 to80 weight %, B₂O₃: 0 to 15 weight %, R₂O (the R=alkali metal element): 0to 20 weight %, Ln₂O₃ (Ln=rare earth element): 3 to 10 weight %, Al₂O₃:0.5 to 15 weight %; and CoO: 1 to 30 weight % on the basis of oxideconversion.
 5. The glass substrate for a magnetic disk according toclaim 1, wherein the rare earth element is selected from the groupconsisting of Gd and Er.
 6. The glass substrate for a magnetic diskaccording to claim 1, wherein the transition metal oxide is CoO.
 7. Theglass substrate for a magnetic disk according to claim 1, wherein saidtexture is formed by irradiating the glass substrate with a laser havinga wavelenght in the range of 450 to 700nm.
 8. A glass substrate for amagnetic disk, comprising: a glass substrate having an informationrecording surface and a non-information recording surface with atexture, which are formed in the surface of the glass substrate, whereinthe glass substrate contains a rare earth element which increases themechanical strength of the glass substrate, the rare earth element beingselected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Er, Tm, Yb and Lu and being contained in an amount of 3 to 10 weight% on the basis of conversion to Ln₂O₃ (Ln=rare earth element), and atransition metal oxide.
 9. The glass substrate for a magnetic diskaccording to claim 8, wherein the transition metal oxide is CoO and iscontained in an amount of 1 to 13 weight % and wherein the wavelengthband ranges from 450 to 700nm.
 10. The glass substrate for a magneticdisk according to claim 8, wherein the surface part of the glasssubstrate is substantially free from a chemical strengthening layer. 11.The glass substrate for a magnetic disk according to claim 8, whereinthe rare earth element is selected from the group consisting of Gd andEr.
 12. The glass substrate for a magnetic disk according to claim 8,wherein the transition metal oxide is CoO.
 13. The glass substrate for amagnetic disk according to claim 8, wherein said texture is formed byirradiating the glass substrate with a laser having a wavelength in therange of 450 to 700nm.